The invention relates generally to overload protection circuitry and more specifically to input overload protection circuitry for a high impedance differential amplifier.
Amplifiers generally, and differential amplifiers more specifically, find extensive application in a wide variety of circuits and systems. High impedance differential amplifiers particularly find application in a large variety of instruments and are commonly referred to as instrumentation amplifiers. Such amplifiers typically receive a small amplitude signal from a transducer or the like and provide the first stage or stages of amplification. In one significant field of application, such instrumentation amplifiers are used to receive and amplify physiological waveforms, such as the electrocardiographic waveforms of a patient. Still further, there may be instances when the input signal exceeds the linear range of the amplifier, as for instance during defibrillation of a patent and/or establishment of electrolyte equilibrium in a patient following the placement of sensing electrodes, and the like. In such instances, it is often desirable and even necessary to prevent such excessive voltages from appearing at the amplifier inputs in order to preserve the integrity of the amplifier. Further, other elements such as CMOS switches and the like in the amplifier's input circuitry may be damaged by the excessive voltages. Accordingly, clamping diodes have been connected across the amplifier input to effectively short circuit excessive signal voltages, thereby protecting the amplifier. Typically, such diode clamping circuits have been entirely passive and have served effectively to short circuit large voltages appearing at the amplifier input. However, that very diode clamping circuit may serve to distort the input signal passed to the amplifier because of some current leakage through the diodes. That leakage, though small, may have a significant effect upon the information signal, particularly where there is a large series resistance in the current path between the patient and the clamping diodes.
Several techniques have evolved for minimizing or preventing leakage currents in clamping networks and tend to vary somewhat with the type of amplifier that's being protected. In addition to purely passive clamping networks, some have applied reverse bias to the diodes which, however, still permits some leakage and resulting distortion to the input signal. Other methods use feedback diodes which require high current drivers to absorb transients. One recent technique which effectively and relatively inexpensively suppresses leakage currents during linear operation is that embodied in the input circuitry of the PULSAR.RTM. 4000 Defibrillator of American Optical Corporation, 36 Crosby Drive, Bedford, Mass. In that instrument, a substantially zero voltage drop across one or more of the diode elements is actively maintained to prevent the flow of leakage currents during normal operation. However, that particular circuit was applicable only to single-ended amplifiers where rejection of common mode voltage is not a problem. Accordingly, it is a principal object of the present invention to provide an input overload protection circuit for a differential amplifier. It is a further object to provide an input overload protection circuit which substantially eliminates the flow of leakage currents during normal linear operation of a high impedance differential amplifier requiring protection.
These and other objects of the invention will be in part obvious and in part pointed out in greater detail hereinafter.